This invention relates to optical interferometers, and particularly to a novel sliding reference interferometer that uses an adaption of Young's two-slit interference for providing a rapid and continuous measurement of the phase profiles and intensity distribution of a high power continuous wave laser beam.
A single laser beam may be considered as a large bundle of individual smaller beams each emitting radiation of substantially the same wavelength but of various intensities and of unrelated phases which may be measured by the use of an interferometer. In conventional prior art, two-beam interferometric phase measurement systems, the intensities and phases of various points in the wave front, are measured with respect to the optical phase and intensity of a preselected reference point in the wave front. In such a system, some of the measured points may be many wavelengths displaced in phase from the reference point. Furthermore, the preselected reference point may have an extremely low intensity thereby making it ineffective for both intensity and phase measurements. Thus, such a prior art system must include means for detecting phase shifts that may be separated by many wavelengths, and the preselected reference point in the wave front must have high intensity and well behaved characteristics. Such a prior art system further requires that the hardware have a very high degree of mechanical stability.
The sliding reference interferometer of the invention does not require a preselected reference point in the wave front being measured, but compares the phase and intensity characteristics of each sampled point on the wave front with those of the adjacent preceding point which, with the proper design, will not be displaced in phase as much as 1/2 wavelength to avoid ambiguity. These individual measurements are phase differential and the phase is determined by summing up the individual elements from an arbitrary reference by means of an appropriate algorithm, which is programmed on a minicomputer. Software has also been developed to display phase and intensity in isometric and isocontour formats.
Briefly described, the sliding reference interferometer of the invention includes an optical system that focuses a plane, such as a laser wave front upon a rotating drum surface containing a spiral array of small aperture pairs that scan the wave front and produce an optical interference fringe pattern according to Young's two-slit interference principles. The interference fringe pattern, which would remain stationary in the absence of any phase shift between adjacent points on the wave front but which is displaced by distances proportional to phase shifts, is focused upon a second surface, which is an optical chopper having equally spaced opaque and transparent sections which alternately blocks and passes the several fringes in the interference pattern as it rotates. A condensing lens behind the chopper integrates the transmitted radiation and directs it to a detector that outputs an electrical sine wave signal at the frequency of the chopper frequency but shifting in phase according to phase variations in the scanned wave front. To measure the phase variations, this electrical sine wave signal is compared, via a high speed (200 MHZ), state of the art, phase comparator, with a second sine wave signal generated by an unscanned reference laser beam that is identically chopped to produce an unvarying signal at the same chopper frequency.